Distributed process control systems, like those used in chemical, petroleum or other process plants, typically include one or more process controllers communicatively coupled to one or more field devices via analog, digital or combined analog/digital buses, or via a wireless communication link or network. The field devices, which may be, for example, valves, valve positioners, switches and transmitters (e.g., temperature, pressure, level and flow rate sensors), are located within the process environment and generally perform physical or process control functions such as opening or closing valves, measuring process parameters, etc. to control one or more processes executing within the process plant or system. Smart field devices, such as the field devices conforming to the well-known Fieldbus protocol may also perform control calculations, alarming functions, and other control functions commonly implemented within the controller. The process controllers, which are also typically located within the plant environment, receive signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices and execute a controller application that runs, for example, different control modules which make process control decisions, generate control signals based on the received information and coordinate with the control modules or blocks being performed in the field devices, such as HART®, WirelessHART®, and FOUNDATION® Fieldbus field devices. The control modules in the controller send the control signals over the communication lines or links to the field devices to thereby control the operation of at least a portion of the process plant or system.
Information from the field devices and the controller is usually made available over a data highway to one or more other hardware devices, such as operator workstations, personal computers or computing devices, data historians, report generators, centralized databases, or other centralized administrative computing devices that are typically placed in control rooms or other locations away from the harsher plant environment. Each of these hardware devices typically is centralized across the process plant or across a portion of the process plant. These hardware devices run applications that may, for example, enable an operator to perform functions with respect to controlling a process and/or operating the process plant, such as changing settings of the process control routine, modifying the operation of the control modules within the controllers or the field devices, viewing the current state of the process, viewing alarms generated by field devices and controllers, simulating the operation of the process for the purpose of training personnel or testing the process control software, keeping and updating a configuration database, etc. The data highway utilized by the hardware devices, controllers and field devices may include a wired communication path, a wireless communication path, or a combination of wired and wireless communication paths.
As an example, the DeltaV™ control system, sold by Emerson Process Management, includes multiple applications stored within and executed by different devices located at diverse places within a process plant. A configuration application, which resides in one or more workstations or computing devices, enables users to create or change process control modules and download these process control modules via a data highway to dedicated distributed controllers. Typically, these control modules are made up of communicatively interconnected function blocks, which are objects in an object oriented programming protocol that perform functions within the control scheme based on inputs thereto and that provide outputs to other function blocks within the control scheme. The configuration application may also allow a process engineer to create or change operator interfaces which are used by a viewing application to display data to an operator and to enable the operator to change settings, such as set points, within the process control routines. Each dedicated controller and, in some cases, one or more field devices, stores and executes a respective controller application that runs the control modules assigned and downloaded thereto to implement actual process control functionality. The viewing applications, which may be executed on one or more operator workstations (or on one or more remote computing devices in communicative connection with the operator workstations and the data highway), receive data from the controller application via the data highway and display this data to process control system designers, operators, or users using the user interfaces, and may provide any of a number of different views, such as an operator's view, an engineer's view, a technician's view, etc. A data historian application is typically stored in and executed by a data historian device that collects and stores some or all of the data provided across the data highway while a configuration database application may run in a still further computer attached to the data highway to store the current process control routine configuration and data associated therewith. Alternatively, the configuration database may be located in the same workstation as the configuration application.
In many distributed process control systems, each field device in the process plant is assigned a unique device tag. The unique device tag provides an easy way to reference the corresponding field device. Device tags may be used during the configuration of the process control system to specify the source or destination, respectively, of an input or output to a function block in a control module. Each signal type has associated with it a particular format or set of information, and the device tag for a particular device may have associated with it a specific signal type such that when the device tag is associated with an input or output of a function block, the function block knows the format and information associated with the signal. In cases in which a field device has multiple signals associated with it (e.g., a valve may measure and transmit both pressure and temperature), device signal tags may be associated with each signal of the field device. During configuration, I/O cards associated with particular field devices are programmed convert data from formats implemented by the controller 40 to formats implemented by the field devices, and vice versa.
Additionally, in many processes, Programmable Logic Controllers (PLCs) are integrated into the process. PLCs traditionally store values from the various field devices to which they are connected in a series of numbered memory registers which, in some cases, are loosely organized, if they are organized at all. The PLC is programmed to control the field devices according to the information in the registers. Integrating the PLCs and associated field devices into the control system is tedious, requiring that each register used in the control strategy programmed into the controllers is individually verified, a process which for large process plants can take months. Additionally, signals brought into the process control network from PLCs do not have the same format or information as a device tag wired directly to the distributed control system (i.e., to the controller via the I/O cards). The same information that would be associated with a single device tag in a configuration that did not include intermediary PLCs may, when coupled to a PLC, be stored in multiple different registers in the PLC. As a result, traditional I/O function blocks do not readily integrate signals from PLCs as I/O inputs, because the information those function blocks expect to receive is not readily provided to the function blocks. Instead, users (e.g., process control plant personnel) are left to configure custom function blocks that retrieve the information from the PLC registers and behave like the device signal tags such that the signals can be used with provided components (e.g., graphical components) that are provided with the process control system. By way of example, a standard graphic for displaying an analog process value output by a field device may expect to receive an output from an analog input (AI) function block that, itself, expects to receive a set of parameters associated with the output from the device (e.g., process value, set point, alarm limits, mode, scale, etc.). When the field device is coupled to the process control system via a PLC, there is no easy way to provide the set of parameters to the function block, as the various ones of the parameters are stored in individual (and often disparate) numerically addressed registers in the PLC.